The technology of cardiac pacemakers has developed to a high level of sophistication of system performance. The current generation of cardiac pacemakers incorporates microprocessors and related circuitry to sense and stimulate heart activity under a variety of physiological conditions. These pacemakers may be programmed to control the heart in correcting or compensating for various heart abnormalities which may be encountered in individual patients. A detailed description of modern cardiac pacemaker technology is set forth in International Application Number PCT/US85/02010, entitled STIMULATED HEART INTERVAL MEASUREMENT, ADAPTIVE PACER AND METHOD OF OPERATION, or U.S. Patent Application Ser. No. 887,297, filed July 18, 1986, entitled PACEMAKER HAVING PVC RESPONSE AND PMT TERMINATING FEATURES assigned to the assignee hereof. The disclosures of those applications are incorporated herein by reference.
It has always been thought that in order to efficiently perform its function as a pump, the heart must maintain a natural AV synchrony. The term "AV synchrony 38 relates to the sequential timing relationship that exists between the contractions of the atria and the ventricles. In a given heart cycle or beat, these contractions are typically manifest or measured by sensing electrical signals or waves that are attendant with the depolarization of heart tissue, which depolarization immediately precedes (and for most purposes can be considered concurrent with) the contraction of the cardiac tissue. These signals or waves can be viewed on an electrocardiogram and include a P-wave, representing the depolarization of the atria; the QRS wave (sometimes referred to as an R-wave, the predominant wave of the group), representing the depolarization of the ventricles; and the T-wave, representing the repolarization of the ventricles. (It is noted that the atria also are repolarized, but this atrial repolarization occurs at approximately the same time as the depolarization of the ventricles; and any electrical signal generated by atrial repolarization is generally minute and is masked out by the much larger QRS-wave on the electrocardiogram.)
Thus it is the P-QRS-T-cycle of waves that represents the natural AV synchrony of the heart. These waves, including the time relationships that exist therebetween, are carefully studied and monitored through conventional ECG techniques whenever the operation of the heart is being examined.
Initiation of the cardiac cycle normally begins with depolarization of the sinoatrial (SA) node. This specialized structure is located in the upper portion of the right atrium wall. For most adults, the SA node depolarizes spontaneously at an intrinsic rate of a little better than once each second (typically about 65-70 beats per minute). The rate of depolarization and, therefore, the heart rate are influenced by various physical factors, which may produce tachycardia or bradycardia depending upon the particular patient condition.
Optimally, in a normal cardiac cycle and in response to the initiating SA depolarization, the atrium contracts and forces the blood that has accumulated therein into the ventricle. A short time later (a time sufficient to allow the bulk of the blood in the atrium to flow through the one-way valve into the ventricle), the ventricle contracts, forcing the blood out of the ventricle to body tissue. A typical time interval between contraction of the atrium and contraction of the ventricle might be 60 ms; a typical time interval between contraction of the ventricle and the next contraction of the atrium might be 800 ms. Thus, it is an atrial contraction (A), followed a relatively short time thereafter by a ventricle contraction (V), followed a relatively long time thereafter by the next atrial contraction, that produces the desired AV synchrony. Where AV synchrony exists, the heart functions very efficiently as a pump in delivering life-sustaining blood to body tissue; where AV synchrony is absent, the heart functions as an inefficient pump (largely because the ventricle is contracting when it is not filled with blood).
Multiple-mode, demand-type, cardiac pacemakers are designed, insofar as is possible, to maintain an AV synchrony for damaged or diseased hearts that are unable to do so on their own. A demand-type pacemaker is one that provides a stimulation pulse only when the heart fails to produce a natural depolarization on its own within a prescribed escape interval. In a dual chamber pacemaker, this is realized by placing electrodes in both the right atrium and right ventricle of the heart. These electrodes are coupled through intravenous and/or epicardial leads to sense amplifiers housed in an implanted pacemaker. Electrical activity occurring in these chambers can thus be sensed. When electrical activity is sensed, the pacemaker assumes that a depolarization or contraction of the indicated chamber has occurred. If no electrical activity is sensed within a prescribed time interval, typically referred to as an atrial or ventricular escape interval, then a pulse generator, also housed within the pacemaker housing, generates a stimulation pulse that is delivered to the indicated chamber, usually via the same lead or electrode as is used for sensing. This stimulation pulse causes or forces the desired depolarization and contraction of the indicated chamber to occur. Hence, by first sensing whether a natural depolarization occurs in each chamber, and second, by stimulating at controlled time intervals each chamber with an external stimulation pulse in the absence of a natural depolarization, the AV synchrony of the heart can be maintained. Thus, with a demand pacer, the heart will either beat on its own (without stimulation from the pacemaker at a rate that is at least just slightly faster than the stimulation rate defined by the escape interval), or the heart will be stimulated by the pacer at a rate controlled by the escape interval. The stimulation rate provided by the pacemaker is typically referred to as the "programmed rate."
Unfortunately, there are many operating constraints and conditions of the heart that complicate the operation of a demand-type pacemaker. For example, there are certain time periods following a depolarization of cardiac tissue (prior to repolarization) when the application of an external electrical impulse is ineffective -- that is, it serves no useful purpose, and thus represents an unneeded expenditure of the pacemaker's limited energy. Therefore the application of stimulation pulses during these time periods is to be avoided.
Further, as demonstrated below, artificially maintaining AV synchrony at high heart rates (e.g., greater than 90 beats per minute) by stimulating both the atrium and the ventricle may not be an efficient way to maintain cardiac output. That is, stimulating the atrium at these high rates may also represent an unneeded expenditure of the pacemaker's limited energy.
Rate responsive pacemakers employ some type of physiological sensor for sensing a change in the metabolic needs of a patient. This sensed change, in turn, is used to adjust the rate at which stimulation pulses are delivered to the heart of the patient by the pacemaker. Thus, as the metabolic needs of the patient increase -- indicating a need for the heart to beat faster -- the rate at which the pacemaker stimulates the heart is increased as a function of this sensed increase in metabolic need. As the metabolic needs of the patient decrease -- indicating a need for the heart to beat slower -- the rate at which the pacemaker stimulates the heart is correspondingly decreased.
In a demand pacer, the physiological sensor (which may be one of numerous types) adjusts the pacing rate by adjusting the escape interval of the pacer. As the escape interval is thus adjusted as a function of sensed physiological need, the rate at which stimulation pulses are provided to the heart -- and hence the heart rate -- is correspondingly varied as a function of sensed physiological need.
Rate-responsive demand pacers may be either single chamber pacers that sense and stimulate in the ventricle (e.g., a VVI mode of operation) at a rate determined by the particular physiological sensor used, or dual chamber pacers that sense and stimulate in both the atrium and the ventricle (e.g., a DDD mode of operation) at a rate determined by the physiological sensor. Patients who are candidates for single chamber rate-responsive pacing usually include patients exhibiting partial or complete heart block. When heart block exists, the ventricle does not consistently follow the atrium, and the needed and desired AV synchrony is lost. Patients who are candidates for dual chamber rate-responsive pacing include this same group of patients (who are candidates for single chamber pacing) plus patients whose atrial contractions are irregular or intermittent.
Heart block, for purposes of this disclosure, means that the stimulus from the SA node -- the heart's natural pacemaker -- is unable to travel to the ventricle to stimulate the ventricle at the appropriate time, i.e., the heart's anteograde conduction path is somehow blocked at least some of the time.
A dual chamber rate-responsive pacer advantageously allows both the atrium and/or the ventricle to be stimulated at a rate commensurate with the sensed physiological need despite an irregular, intermittent, or non-functioning S-A node. Disadvantageously, operation of a dual chamber pacemaker, when providing stimulation pulses to both the atrium and the ventricle, expends significantly more power than a single chamber pacemaker, thereby shortening the useful life of the pacemaker's batteries. As indicated more fully below, while dual chamber rate-responsive pacing may be very beneficial at lower heart rates, single chamber rate-responsive pacing may be more than adequate to maintain cardiac output at higher heart rates. Thus, what is needed is a dual chamber rate-responsive pacer that conserves power by automatically switching to a single chamber mode of operation at higher heart rates.
A single chamber rate-responsive pacer (or a dual chamber pacer operating in a single chamber mode) advantageously allows the ventricle to be stimulated at a rate commensurate with the sensed physiological need despite a completely, intermittently, or partially blocked anteograde conduction path.
It has recently been discovered that many patients who exhibit partial, intermittent, or complete heart block at normal heart rates, e.g., 70 beats per minute (bpm), will exhibit normal anteograde conduction at higher rates, e.g., 110-120 bpm. Thus, if these patients are fitted with a conventional VVI pacer, or a dual chamber pacer programmed to operate in a VVI mode, such pacer provides ventricular stimulation as required at normal heart rates as defined by the pacemaker's programmed rate. Disadvantageously, however, natural AV synchrony is lost whenever the pacer provides a stimulation pulse to the ventricle. If this patient (with a conventional VVI pacer and exhibiting partial, complete or intermittent heart block only at the lower normal heart rates) exercises, and assuming the patient's SA node functions normally, the SA node attempts to make the heart beat faster as the physiological needs resulting from the exercise increase. As long as heart block exists, however, such attempts are ineffective, and the pacer will continue to provide ventricular stimulation pulses at the programmed rate. At some point (which will vary from patient to patient), as the patient continues to exercise, the natural conduction path is restored, and the ventricle is stimulated from the SA node (i.e., heart block no longer is present), and natural AV synchrony is advantageously restored. The result is that the patient, having his or her natural AV synchrony restored, feels great.
After exercise, when the heart beat rate returns to normal levels, heart block returns, and the VVI pacer again takes over stimulating the ventricle at the programmed rate. Natural AV synchrony is lost. The patient typically feels all right, but not as good as when natural AV synchrony was present.
If a rate-responsive VVI pacemaker is employed, or a rate-responsive dual chamber pacemaker is programmed, or otherwise switched, to operate in the VVI mode, the physiological sensor used therewith senses the increased physiological need brought on by the patient's exercise. This causes the pacing interval (referred to herein as the escape interval) of the rate-responsive pacemaker to be adjusted accordingly. As long as heart block exists, this presents no problem (and, in fact, the rate-responsive pacemaker continues to perform its intended function). However, should heart block cease, then the ventricle is stimulated from the SA node through the natural anteograde conduction path and AV synchrony should, in theory, return. Unfortunately, because the basic pacing or escape interval of the rate-responsive pacer is also changing (being adjusted in accordance with the sensed physiological need), it is possible and quite probable that competition will exist between the SA node and the rate-responsive pacemaker. Such competition may result when the programmed rate of change of the VVI pacemaker does not match the rate of change of the heart's SA node. Thus, an R-wave may not be sensed because it does not fall within a shortened escape interval of the rate-responsive pacemaker. Conversely, an R-wave may not be sensed because it occurs prior to the termination of a pacemaker-defined refractory period. In either event, AV synchrony can be lost.
What is needed, therefore, is a rate-responsive pacemaker that prevents competition between the rate-responsive pacemaker and the heart's SA node should the anteograde conduction path be restored. Such a rate-responsive pacemaker is realized, according to the teachings of the present invention by providing hysteresis. A programmable cardiac pacemaker with hysteresis is disclosed in U.S. Pat. No. 4,263,915 (McDonald et al). As indicated in that patent, the concept of hysteresis as a technique of cardiac pacing is well-known in the prior art. According to the patent disclosure, the hysteresis concept is introduced into a pacemaker which is generating artificial stimulating pulses at a constant rate. However, the disclosure of that patent does not extend to the provision in the present invention of utilizing the hysteresis concept in a pacemaker of the rate-responsive type.